Silicon carbide has been a perennial candidate for use in the manufacture of semiconductor electronic devices. Silicon carbide has a number of characteristics which make it theoretically advantageous for such uses. These include a wide band gap, a high thermal conductivity, a low dielectric constant, a high saturated electron drift velocity, a high breakdown electric field, and a high melting point. Taken together, these properties indicate that semiconductor devices formed from silicon carbide should be operable at much higher temperatures than devices made from other semiconductors, as well as at higher speeds at higher power levels, and with an increased device density.
Nevertheless, semiconductor electronic devices made from silicon carbide have yet to make a viable appearance in any circumstances other than laboratory research and have yet to reach their commercial potential. This lack of success results, at least partially, from the difficulty encountered in working with silicon carbide. It is an extremely hard material, often used as an abrasive. It often must be worked at extremely high temperatures under which other materials cannot be worked, and from a semiconductor standpoint, crystallizes in well over 150 polytypes, many of which are separated by rather small thermodynamic differences. For these latter reasons, production of monocrystalline thin films of silicon carbide that are necessary for certain devices and production of large single crystals of silicon carbide which are useful for other applications, has remained an elusive goal. Additionally, certain doping techniques which have been successfully developed for other materials have proved unsuccessful when used in connection with silicon carbide. In particular, the successful use of ion implantation techniques has, until recently, remained unachieved.
Recently, however, a number of developments have occurred which have successfully accomplished both single crystal and thin film growth of silicon carbide, as well as successful ion implantation techniques. These are included in several copending patent applications which have been assigned to the assignee of the present invention, and the contents of which are incorporated herein by reference. These include: Davis, et al., "Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon", Ser. No. 113,921, filed Oct. 26, 1987 now U.S. Pat. No. .varies.,912,063; Davis et al., "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 113,573, filed Oct. 26, 1987 now U.S. Pat. No. 4,912,064; Davis et al., "Sublimation of Silicon Carbide to Produce Device Quality Single Crystals of Silicon Carbide," Ser. No. 113,565, filed Oct. 26, 1987; and Edmond et al., "Implantation and Electrical Activation of Dopants Into Monocrystalline Silicon Carbide," Ser. No. 113,561, filed Oct. 26, 1987 now U.S. Pat. No. 4,866,055.
The success of these techniques has made possible improved techniques for producing p-n junctions and junction diodes in silicon carbide.
As is known to those familiar with electronic devices, the production of p-n junctions is a fundamental step in both characterizing a semiconductor material and in fabricating other junction devices such as diodes, transistors and the like. Accordingly, given the theoretical advantages of silicon carbide and the necessity of producing junctions to develop devices, there has been significant interest in methods of producing such junctions in silicon carbide. Most of these efforts, however, have not attempted to use ion implantation techniques, but instead have developed methods of producing what may be referred to as "fused" junctions. In such junctions, alternating regions of p-type and n-type silicon carbide are formed in contact with one another to form the p-n junction. Typical techniques have included melting a dopant metal directly on the surface of silicon carbide so that some of the dopant dissolves into the silicon carbide to produce an oppositely doped region, the border of which forms the p-n junction. Others use separately formed portions of p-type and n-type silicon carbide and fuse them to one another using various processes to form the p-n junction. Yet another technique is to form an epitaxial growth of p or n-type silicon carbide upon a substrate of silicon carbide having the opposite conductivity type. Finally, there are a number of other methods such as the various solvent based techniques.
Less interest and very little success have occurred where ion implantation techniques have been attempted upon silicon carbide. Ion implantation is a doping technique in which the desired impurities (dopants) are introduced into the semiconductor lattice by bombarding the surface with high energy dopant ions. This technique offers more control over dopant levels and locations than most of the other doping techniques.
Kalinina et al, Electrical Prooerties of P-N Junctions Formed By Ion Implantation In N-Type SiC. Sov. Phys. Semicond. 12, 1372 (1978), discuss ion implantation of silicon carbide aluminum ions at room temperatures, followed by annealing at about 1800.degree. C. Marsh and Dunlap, Ion-Implanted Junctions and Conducting Layers in SiC, Radiation Effects 1970, Vol. 6, p. 301, discuss ion implantation in silicon carbide to produce junctions and diodes in which nitrogen, phosphorous, antimony, and bismuth were all implanted at room temperature, and in which antimony was additionally implanted at 500.degree. C., all to produce n-type conductivity. Similarly, boron, aluminum, gallium, and thallium were implanted at room temperature in an attempt to obtain p-type conductivity. According to Marsh, however, the only successful results were in producing donor (n-type) implanted materials and Marsh admits "little or no success in creating p-type SiC . . . using implant and anneal procedures similar to those used for the n-type dopants."
Accordingly, there presently exists no successful technique for ion implantation of both donor and acceptor dopant ions into monocrystalline silicon carbide which results in appropriately electrically activated n and p-type materials.
Therefore, it is an object of the present invention to provide a method of forming a diode operable at high temperatures, high power levels and under conditions of high radiation density which comprises forming a region of monocrystalline, electrically activated doped silicon carbide having a first conductivity type on a substrate of monocrystalline doped silicon carbide having the opposite conductivity type by high temperature ion implantation of doping ions into the doped silicon carbide substrate which give the ion implanted region the first conductivity type.
It is another object of this invention to successfully form a region of monocrystalline, electrically activated, p-type silicon carbide on a substrate of monocrystalline doped silicon carbide having an n-type conductivity type.
It is a further object of the invention to provide a method of forming a diode having a region of monocrystalline, electrically activated doped silicon carbide having a p-type conductivity on a substrate of monocrystalline doped silicon carbide having an n-type conductivity by high temperature ion implantation of group III doping ions into the doped silicon carbide substrate.
It is another object of the invention to provide a planar diode operable at high temperatures, high power levels and under conditions of high radiation density in which the diode comprises a portion of silicon carbide having a first conductivity type, a portion of more heavily doped silicon carbide having the opposite conductivity type immediately adjacent the portion of first conductivity type and respective ohmic contacts on each of the respective portions of silicon carbide.
It is a further object of the invention to provide a mesa diode operable at high temperatures, high power levels and under conditions of high radiation density which diode comprises a portion of silicon carbide having a first conductivity type, a portion of more heavily doped silicon carbide having the opposite conductivity type positioned upon the first portion of silicon carbide, and respective ohmic contacts to the respective portions of silicon carbide.
It is another object of the invention to provide a planar diode which comprises a doped silicon carbide substrate having a first conductivity type, a doped well of silicon carbide in said silicon carbide substrate, said doped well having the opposite conductivity type from said doped silicon carbide substrate, an insulation layer on the silicon carbide substrate surface and positioned over the p-n junction between the well and the substrate at the surface, and a conductor positioned upon the insulation layer directly above the p-n junction between the well and the substrate for permitting a potential to be applied directly above the p-n junction to thereby confine the depletion zone from expanding in a direction parallel to the surface of the diode.